The present invention relates to a blow-molding unit for synthetic plastic materials.
Hollow bodies of synthetic plastic materials can be produced, inter alia, by blow molding. If they are produced in this manner, they are usually manufactured in partable mold sections from an extruded preform, in such a manner that an overpressure is produced in the interior of the preform to expand the latter into engagement with the walls of the mold sections which bound an internal mold cavity. As a general rule it is desired that the mold sections be replaceable for repair or other reasons. Because of this, the mold sections are usually mounted on supporting plates. The closure mechanism which moves the supporting plates with the mold sections into mold-closing position or out of the same, engages special closure plates on which the support plates are mounted, or on which the mold sections themselves are mounted directly.
In more recent developments of the blow molding technique a two-stage or multi-stage approach is utilized in which a form composed of mold sections in open position is placed at a receiving station and has admitted into it a preform from the outlet nozzle of an extruder. The form is then closed around the preform and is moved to a blowing station where the preform is expanded to the desired shape. During this time the extruder nozzle extrudes another preform in tubular or other configuration in preparation for a new working cycle. This means that the blow mold composed of the mold sections operates both in a receiving station and in a blowing station and shuttles between these two stations.
It is already known to increase the strength of a blow-molded body of synthetic plastic material, without requiring additional quantities of material, by subjecting the article to so-called "biaxial stretching". When this approach is used, it is known to use a premold in which the preform is shaped to approximately the size and configuration of the desired final article, and wherein this preform is then shaped in a final mold to the final shape and configuration desired for the article. This is, for example, disclosed in German Offenlegungsschrift No. 2,354,214.
As a rule, the two mold sections of a blow mold are not separately driven. It is customary to use guide members which usually carry one of the support or closure plates rigidly connected to them, whereas the other support or closure plate is slidable along the guide members toward and away from the first-mentioned support or closure plate. Each of the plates, of course, can raise one of the mold sections. To obtain an exact guidance for the mold sections it has previously been customary to use four guide members which extend through the corners of the respective support plates or closure plates. If the mold sections are moved in a horizontal or inclined path, which may even be slightly curved, this brings with it the disadvantage that at least one of the guide members will always be in the way when the opened mold section is moved from the blow molding position to the receiving position beneath the extruder nozzle from which already a length of plastic tubing or the like extends as a preform.
To overcome this disadvantage it is already known from the prior art to eliminate at least one of the guide members, namely the one which is located uppermost relative to the direction of movement of the mold sections towards the extruded preform. During the movement of the mold from the blowing position to the preform receiving position, this omitted guide member then cannot be in the way, so that despite the fact that a portion of extruded synthetic plastic material constituting the preform hangs from the extruder nozzle, the mold sections of the blow mold can move to the proper receiving position, also known as the mold charging position. It has also become known to provide mounting plates or closure plates carrying the mold sections, wherein two diagonally opposite corners are provided with the guide members, the corners being so selected that the aforementioned interference with the movement does not occur.
However, if the blow mold is a premold in which a preform is first expanded at a premolding station, which must then be received in a final mold and further expanded to the finished article, the guide member which is located remote from the extruder nozzle would not push away the extruded preform hanging from the extruder nozzle during the movement of the mold from the blowing position to the charging position, but the other upper guide member would during this movement impact against the premold which is produced at the charging or premolding station and which depends from the blowing mandrel in readiness for engagement and transfer to the final blowing station. The same is true of the so-called directed transfer of a blow molded hollow body in the final blow molding station, in which for example the hollow body is moved downwardly by the blow molding mandrel after the final mold is open, and is placed onto a stationary or movable supporting surface, as for instance disclosed in German Offenlegungsschrift No. 2,355,114.
It will be appreciated that in the latter case there will be a longer time before the blowing mandrel has been withdrawn sufficiently far into its readiness position for the next-following blowing cycle, that it is entirely withdrawn out of the path of movement of the blow mold. During this time, the open blow mold would have to remain stationary at the final blowing station. This is a disadvantage in terms of the production capabilities and to overcome this disadvantage it is known to utilize for the opening and closing movements of the sections of the blow mold only two guide members which are located outside and below the support or closure plates of the mold, via which the mold sections are guided directly or indirectly by means of the supporting plates, as disclosed in German Offenlegungsschrift No. 2,321,694. However, in that case after the mold sections have moved to closed position, they will not be part of a force system which is in such equilibrium that it can properly withstand the internal pressure that is applied when the article inside the mold sections is expanded. In other words, in such a set-up there is the danger that due to the pressure which develops during the expansion of the article in the mold composed of the mold sections, the mold sections will be urged apart and the guide members for them might become bowed or buckled. This means that the guide members must be made exceptionally strong in relationship to the other components.